When gluing wood, it is common to use an adhesive system based on a curable resin, such as a formaldehyde resin, which, for example, can be an amino resin or a phenolic resin. Important properties of an adhesive include adhesive strength, curing time and temperature, water-resistance and emission of formaldehyde.
High water-resistance is especially required for some glued wooden products, such as laminated beams, plywood, and other products for outdoor use. Melaminic amino resin adhesives, and also phenolic resin based adhesives, are commonly used when high water-resistance is required. The advantages of using amino resin based adhesives over phenolic resin based ones are, for example, a much less coloured bondline and less environmental impacts since some phenolic resins need paraformaldehyde to cure.
Melaminic amino resins are condensates of carbonyl compounds, such as aldehydes, with melamine, or a combination of melamine with other compounds containing amino, imino or amide groups. The most common melaminic amino resins are condensates of formaldehyde and melamine alone, or melamine and urea giving melamine-formaldehyde (“MF”) and melamine-urea-formaldehyde (“MUF”). A MUF resin can also be made by mixing an MF resin and a UF resin. Melaminic amino resins are usually cured by using acidic hardener compositions.
Emission of free aldehyde, particularly formaldehyde, from adhesives based on melamine based resins and phenolic based resins is a growing concern. Formaldehyde is to various extent present in formaldehyde based melaminic amino resins as free formaldehyde but also further emitted from the resins during curing. This gives environmental problems, both before curing during handling and application of the resin, and emission problems from the finished products after curing. Phenolic resins may also emit formaldehyde during curing and during handling of the resin.
JP Laid Open No. 1977-51430, discloses an adhesive composition for manufacturing plywood comprising a melamine resin and a resol-type phenolic resin. However, there is nothing mentioned about any reduction of formaldehyde emission.
Aldehyde emission from an adhesive system can be reduced by using various additives which act as catchers for the aldehyde. However, such additives do not generally themselves possess any adhesive properties. Thus, the quality of the adhesive bond may be negatively effected. Furthermore, these additives may take part in unwanted reactions with other components of the adhesive system, and addition of such additives also often creates a more complex formulation procedure.
Thus, it is desirable to provide a method of gluing wood based materials, and an adhesive system, which gives low emission of aldehyde, and high quality adhesive bonds.
Therefore, it is an object of the present invention to provide a method for gluing wood based materials which gives low emission of aldehyde. It is another object of the present invention to provide an adhesive system, as well as a phenolic resin composition intended for use in an adhesive system, which gives low emission of aldehyde. Finally, it is an object of the present invention to provide a wood based product which gives low emission of aldehyde.